Firstbootprompt: remove flags from languages.

http://www.flagsarenotlanguages.com

Add some other icons to make the window a bit more spicy as suggested by
Axel.

Change-Id: I859cefc7f5cb99f9a5465902c9d471f0b9857b71
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2527
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

Renamed ReadOnlyBootPrompt to FirstBootPrompt.

The previous name is no longer applicable, as *BootPrompt is now launched on
the first boot of writable medium (e.g., USB thumb drive and hard drives).
Fixes part of #9399.

Notes:
* BootPrompt.rdef may have lost its history.
* The renaming was done with the following command:
git mv src/apps/readonlybootprompt src/apps/firstbootprompt

Renamed ReadOnlyBootPrompt to FirstBootPrompt.

The previous name is no longer applicable, as *BootPrompt is now launched on
the first boot of writable medium (e.g., USB thumb drive and hard drives).
Fixes part of #9399.

Notes:
* BootPrompt.rdef may have lost its history.
* The renaming was done with the following command:
git mv src/apps/readonlybootprompt src/apps/firstbootprompt